It's back to the drawing board for astronomers and astrophysicists trying to explain why the supermassive black holes at the hearts of some galaxies pump out huge amounts of radiation. Such an "active galactic nucleus" (AGN) presumably arises when ultrahot gas falls into a galaxy's central black hole, and common wisdom held that the matter is tipped into the black hole when galaxies collide. However, a painstaking visual inspection of 1400 remote galaxies suggests that black hole activity is unrelated to galactic collisions and mergers, at least for the past 7.5 billion years of cosmic history.
Most major galaxies harbor supermassive central black holes. For instance, the black hole in our own Milky Way galaxy is 4.3 million times as massive as the sun. In other galaxies, black holes may weigh in at hundreds of millions or even a few billion solar masses. Most of these hungry monsters are quiet and fairly inconspicuous, but others spew out energetic particles, high-energy x-rays, and visible light. Such high-profile black holes are believed to be actively gobbling up huge loads of interstellar gas, which is heated to millions of degrees just before plunging over the edge.
But what fuels black holes in the first place? Many astronomers think galactic interactions play a major role in feeding the central gluttons. When two galaxies collide or merge, they become warped and distorted by tidal forces, and gas clouds—or even complete stars—can be funneled into the core.
That scenario sounds sensible, but it's wrong, according to a large team of astronomers led by Knud Jahnke and Mauricio Cisternas of the Max Planck Institute for Astronomy in Heidelberg, Germany. "There is no evidence that major merging plays a key role in the triggering of [black hole] activity," they claim in the 10 January issue of The Astrophysical Journal.
The researchers used a straightforward technique to arrive at their surprising conclusion. Using data from the European Space Agency's orbiting XMM-Newton x-ray observatory, they selected 140 galaxies with AGNs, indicating the presence of feasting black holes. Then they added a large control sample of 1264 nonactive galaxies at similar distances, between 3.5 billion and 7.5 billion light-years from Earth. Finally, 10 team members inspected Hubble Space Telescope optical images of the 1404 galaxies and sorted them into three categories: not distorted, mildly distorted, and strongly distorted. The "jury" didn't know which galaxies were active and which ones were quiet, as the Hubble images had been processed to hide the telltale bright cores.
Surprisingly, 85% of the active galaxies were classified as not being strongly distorted, suggesting that they hadn't experienced major collisions or mergers in their recent past. Moreover, the fraction of mildly and strongly distorted systems turned out to be more or less the same for the sample of 140 active galaxies as for the control sample of quiet galaxies. According to the authors, "mergers and interactions involving [active galaxies] occur no more frequently than for inactive galaxies."
"It's a nice piece of work," says astronomer Huub Röttgering of Leiden Observatory in the Netherlands, who specializes in active galaxies, "but it's not completely unexpected." Earlier studies had hinted at the same conclusion, says Röttgering. "There's a general consensus that the very brightest active galactic nuclei are the result of major mergers," he says, "but for the run-of-the-mill AGNs, other processes might be more important."
Those other processes include collisions of giant gas clouds within galaxies, internal instabilities, tidal interactions during flybys of smaller galaxies, and minor mergers that don't produce conspicuous distortions. Which process tops the list is unknown, says Röttgering: "This result indicates that we have to study much harder on the alternatives."